A radial multi-piston pump (DE-OS No. 35 22 796, for example) of the kind used herein has a pressure limiting valve that is coaxial with the drive shaft and is accommodated in an intermediate cover. The pressure limiting valve protects the pump and a consuming device connected with it against overloading after a maximum pressure determined by a spring is reached. The upstream high-pressure side of the pressure limiting valve is exposed to a collecting chamber that receives the oil under pressure from all pistons. The outlet side of the pressure limiting valve is connected with a return chamber that is closed by the front surface of the drive shaft.
Impact noises develop when such a radial piston pump is cold started. If the driver turns an auxiliary power steering mechanism supplied by the pump to highest pressure to get out of a parking space, the permissible maximum pressure builds up and the pessure limiting valve opens. This lets the volume being delivered by the pump at the moment to flow back to the return chamber. A back pressure is developed in the return chamber by the still viscous oil that, because of necessary shaft axial play, suddenly moves the drive shaft against the pump casing. This is the cause of the impact noises. The possibility of letting the oil flow off to the suction side of the pump more quickly through several drilled holes, instead of only one, does exist. However, a resulting unfavorable cost of production results. Also, the oil mass that is present in the entire pump and in the suction line acts as retardant to flow. As a result, the pump can only suck in a small volume initially when the oil is cold.
The objects of the invention are to eliminate the impact noises caused by a cold start; improve the pump's suction efficiency, improve the bearing lubrication efficiency while the pump is warming up by warming the return oil; maintain a low manufacturing cost, all effected by the main feature of the invention, viz., to warm return oil on start up.